Experimental implementation for webapp plugins (#7185)

* Start of experimental implementation for webapp plugins

* Updates to webapp plugin architecture

* Update pluggable test

* Remove debug code

webapp/plugins/index.js

@@ -0,0 +1,51 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+// EXPERIMENTAL - SUBJECT TO CHANGE
+
+import store from 'stores/redux_store.jsx';
+import {ActionTypes} from 'utils/constants.jsx';
+import {getSiteURL} from 'utils/url.jsx';
+
+window.plugins = {};
+
+export function registerComponents(components) {
+    store.dispatch({
+        type: ActionTypes.RECEIVED_PLUGIN_COMPONENTS,
+        data: components || {}
+    });
+}
+
+export function initializePlugins() {
+    const pluginJson = window.mm_config.Plugins || '[]';
+
+    let pluginManifests;
+    try {
+        pluginManifests = JSON.parse(pluginJson);
+    } catch (error) {
+        console.error('Invalid plugins JSON: ' + error); //eslint-disable-line no-console
+        return;
+    }
+
+    pluginManifests.forEach((m) => {
+        function onLoad() {
+            // Add the plugin's js to the page
+            const script = document.createElement('script');
+            script.type = 'text/javascript';
+            script.text = this.responseText;
+            document.getElementsByTagName('head')[0].appendChild(script);
+
+            // Initialize the plugin
+            console.log('Registering ' + m.id + ' plugin...'); //eslint-disable-line no-console
+            const plugin = window.plugins[m.id];
+            plugin.initialize(registerComponents, store);
+            console.log('...done'); //eslint-disable-line no-console
+        }
+
+        // Fetch the plugin's bundled js
+        const xhrObj = new XMLHttpRequest();
+        xhrObj.open('GET', getSiteURL() + m.bundle_path, true);
+        xhrObj.addEventListener('load', onLoad);
+        xhrObj.send('');
+    });
+}